a canvas implementation for DesktopX

Tuesday, August 26, 2008 by Littleboy | Discussion: DesktopX


This small update brings a proper shadow implementation with blurring, two new functions (textPath and textAlongPath) and a few bug fixes. textAlongPath is still somewhat buggy with multi-segment paths but otherwise works great (see the TextWave demo). I fixed a few problems with arcTo that wasn’t working as specified and made a few changes here and there. More importantly, I fixed a nasty crash bug that was sometimes happening on exit.

There is also two new utility functions available from script allowing you to suspend the canvas drawing updates. This allows you to do all the drawing necessary for a frame of animation and refresh the canvas at the end, instead of having it refresh at 30fps while your script is doing all the drawing operations.

With this new API, it was possible to fix all the flickering in the included examples! I’ve also added a few new example scripts. Check out the Polygon example, as well as all the text-related ones.



Every major browser has support for it, Yahoo! Widgets has support for it, how come there isn't something for DesktopX?

Enter DXCanvas, a DesktopX drawing plugin that implements the Canvas spec (plus some DesktopX-specific additions).

How does it work

Canvas Controller

DXCanvas is a DesktopX drawing plugin, which means it takes over drawing for the object it is associated to.

In the configuration, you can set the initial size for the canvas. By default, the drawing surface size will be 300x150. You can also specify whether you want the surface to be transparent and show the other objects or windows under it, or opaque.

A canvas object is made available to scripting. To be able to draw on the canvas, you need to request a context. This is done by calling getContext(type) on the canvas object. The only supported type right now is "2d".

See the list of functions in the implementation section. Note that some are slightly different from the Canvas specification, or are completely new. This is to adjust to differences in what is possible in a browser and in DesktopX.

Canvas Controller Widget

I've made a little widget to make testing easier. It loads a list of scripts from a user-defined folder and allows you to switch between them. It support scripts written in both JScript and VBScript.

It basically associate the script to an object having the DXCanvas plugin as a capability.

Two functions are required for the scripts to work: Object_OnScriptEnter() and Object_OnScriptExit().

The canvas size is reset its default size of 300x150 when switching between scripts, so make sure to set it to the required size if needed by your script.

To add a new script to the list, create a new .js or .vbs file in a folder and select this folder in the widget preferences. A few test scripts are included in the Script folder.

It also support subfolders (only 1 folder deep), so you can organize your scripts in subfolders.

How to help

Download the Canvas Controller widget and start creating scripts!

What to look for

  • strange behavior: colors are not right, context is not saved or restored correctly, etc.
  • the output from a script is different in Firefox or Safari
  • crashes or memory leaks

What is broken/not working properly

  • Drawing still flicker (not as badly as before)
  • This is a DEBUG build, so it's going to be slower than normal



1.1 Build 287:

  • Added manual drawing mode with suspendDrawing/resumeDrawing methods
    (this should allow users to work around the flickering)
  • Fixed text position being off baseline when drawing on a path
  • Fixed a rare crash when drawing text on a path
  • Activated Pixman MMX&SSE2 fast paths
  • Updated Pango to fix font leak (rev 2746)
  • Updated Pixman (0.13.2)

1.1 Build 269:

  • Added textPath, textAlongPath (with stroke or fill option)
  • Added support for shadows
  • arcTo is incorrect when the three points are collinear
  • arcTo should draw a straight line from P0 to P1
  • restore() with an empty stack has no effect
  • Corrected order of data returned by getImageData (BRGA -> RGBA)
  • ImageData is now converted to non-premultiplied color
  • Preserve the current path when clipping
  • rgba colors were not converted to string properly (green had the wrong value)
  • Some composite operations are now handled correctly
  • Fixed Debug build crashing when expired
  • Fixed stack overflow crash on exit
  • Added check for font validity on assignment and fallback to default font (will raise an error in debug mode if font is incorrect)
  • Moved state management to its own class
  • Updated Cairo(1.8.4), Pixman(0.12.0), Pango(1.22.3), Glib(2.18.3) and libpng(1.2.33)

1.0 Build 225:

  • Fixed large canvas objects taking a lot of CPU
  • Log is always enabled

1.0 Build 217

  • Fixed drawing being incomplete in some cases
  • Added workaround for pink color drawing transparent. DesktopX always use pink to draw transparent surfaces, even in per-pixel mode. rgb(255, 0, 255) will be changed to rgb(255, 1, 255)

1.0 Build 214

  • Added per-object log files
  • Added more log info when creating a new context object

1.0 Build 211

  • Added support for % in rgb/rgba colors
  • Added "transparent" color (equivalent to "rgba(0,0,0,0)")
  • Added SVG colors "darkgrey", "darkslategrey", "dimgrey", "grey", "lightgray", "lightslategrey" and "slategrey"
  • Zero size canvas is allowed
  • Properly share canvas state (context style stack was not properly shared before)
  • DrawImage and DrawImageRegion check for negative width and height and adjust the coordinates
  • Only match full color strings
  • Accept rgb values outside [0;255] and clamp the value properly
  • Clamp alpha and use premultiplied color values when painting with alpha (still somewhat wrong)
  • Fixed leak and crash with CanvasImageData & CanvasPixelArray
  • Fixed crash with invalid parameters in createImageData and getImageData
  • Zero-length line gradients should paint nothing
  • Radial gradients with the same start and end circle should paint nothing
  • Image data now accepts negative width and height (you get the untransformed data for the resulting rectangle)
  • arc() with zero radius draws a line to the start point.
  • arcTo() has no effect if the context has no subpaths or if P0 = P1
  • arcTo() draws a straight line to P1 if P1 = P2 or if radius = 0
  • bezierCurveTo() has no effect if the context has no subpaths
  • lineTo() has no effect if the context has no subpaths
  • quadraticCurveTo() has no effect if the context has no subpaths
  • Updated cairo, pixman and pango libraries

1.0 Build 201

  • Added hsl/hsla color parsing
  • Added some logging (DXCanvas.log in object directory)
  • Fixed text stroking always drawing at position (0,0)
  • Fixed text baseline being off by the text height

1.0 Build 191

  • Added redraw calls buffering (less flickering)
  • Added debugMode attribute to canvas. In debug mode, passing invalid parameters will result in an error being raised instead of the invalid value being silently ignored.
  • No longer returns errors when passing invalid parameters to a lot of functions (as specified in Canvas specification)
  • CanvasPixelArray.XXX6(index, value) now takes an int instead of a char and will clamp that value to [0;255]

1.0 Build 180

  • Implemented toImage
  • Implemented createImageData, getImageData and putImageData
  • Added canvas support to createPattern, drawImage and drawImageRegion
  • Fixed createPattern, drawImage and drawImageRegion not working in VBScript
  • Fixed createPattern, drawImage and drawImageRegion not making a copy of the source image or canvas
  • Fixed drawImageRegion using wrong default values for dw and dh
  • Fixed drawImageRegion not creating a new path, resulting in image "corruption"

1.0 Build 168

  • Implemented font attribute setter/getter
  • Implemented fillText, strokeText and measureText (using Pango)
  • Implemented arcTo (using patch from Behdad Esfahbod)
  • Fixed image loading (SD_LOAD_IMAGE seems to be corrupting some images...)

1.0 Build 159

  • Implemented textAlign and textBaseline attributes setters/getters  (not used yet)
  • Implemented createPattern, loadImage, drawImage and drawImageRegion
  • Update to cairo 1.7.4 (better support for text)
  • Statically linked to cairo lib (no need for cairo.dll anymore)

1.0 Build 149

  •  First test version


You can download a test version here. It only includes the Canvas Controller widget for now. Please do not use the DXCanvas plugin in your own objects and widgets yet. This version of the plugin will expire on the 10/01/2008.


Under the hood, it's using Cairo, a 2D vector graphics library that also powers the Mozilla and Yahoo Widgets implementations. Right now, the Cairo library is dynamically loaded at runtime, but I hope to have it statically linked into the plugin for the final version.

Here is a list classes with their attributes and functions and the state of their implementation


width YES  
height YES  
debugMode YES Invalid input will raise an error instead of being ignored.
getContext(type) YES  
toImage(path) YES Saves to a PNG file (replaces toDataURL)
suspendDrawing() YES  
resumeDrawing() YES  



globalAlpha YES  
globalCompositeOperation YES  
strokeStyle YES  
fillStyle YES  
lineWidth YES  
lineCap YES  
lineJoin YES  
miterLimit YES  
shadowOffsetX YES Shadows are not implemented yet.
shadowOffsetY YES Shadows are not implemented yet.
shadowBlur YES Shadows are not implemented yet.
shadowColor YES Shadows are not implemented yet.
font YES  
textAlign YES  
textBaseLine YES Hanging and ideographic baselines are treated as alphabetic.
canvas YES  
save YES  
restore YES  
scale(x, y) YES  
rotate(angle) YES  
translate(x, y) YES  
transform(m11, m12, m21, m22, dx, dy) YES  
setTransform(m11, m12, m21, m22, dx, dy) YES  
createLinearGradient(x0, y0, x1, y1) YES  
createRadialGradient(x0, y0, r0, x1, y1, r1) YES  
createPattern(input, repeat) YES  
clearRect(x, y, w, h) YES  
fillRect(x, y, w, h) YES  
strokeRect(x, y, w, h) YES  
beginPath YES  
closePath YES  
moveTo(x, y) YES  
lineTo(x, y) YES  
quadraticCurveTo(cpx, cpy, x, y) YES  
bezierCurveTo(cp1x, cp1y, cp2x, cp2y, x, y) YES  
arcTo(x1, y1, x2, y2, radius) YES  
rect(x, y, width, height) YES  
arc(x, y, radius, startAngle, endAngle, anticlockwise) YES  
fill() YES  
stroke() YES  
clip() YES  
isPointInPath(x, y) YES  
fillText(text, x, y) YES  
strokeText(text, x, y) YES  
measureText(text) YES  
textAlongPath(text, stroke) YES Adds (or draws) the specified text along the current path
pathText(text) YES Adds the strokes from the specified text to the current path
loadImage(path) YES  
drawImage(input, dx, dy, dw, dh) YES  
drawImageRegion(input, sx, sy, sw, sh, dx, dy, dz, dh) YES  
createImageData(sw, sh) YES  
getImageData(sx, sy, sw, sh) YES  image data uses the BGRA format
putImageData(imageData, dx, dy, dirtyX, dirtyY, dirtyWidth, dirtyHeight) YES  



addColorStop(offset, color) YES  



width YES  
height YES  
data YES  



length YES  
XXX5(index) YES  
XXX6(index, value) YES  



width YES  


First Previous Page 1 of 5 Next Last
Reply #1 Tuesday, August 26, 2008 7:34 AM
Great job, work fine!
Reply #2 Tuesday, August 26, 2008 10:43 AM
Bam! Excellent work!!!
Reply #3 Tuesday, August 26, 2008 12:04 PM

When implemented, would it be possible, using one of the image handling functions (i.e. getImageData or createImageData, or the ImageData 'data' value ), to draw to a 'hidden' context and then set the object.picture property of a regular DX object with the data in the context? This would allow a DX object to have a 'dynamic' image, not based on pre-made .PNGs or states.

Thanks for developing this. This adds a LOT of power and opens a bunch of posibilities for DX.
Reply #4 Tuesday, August 26, 2008 12:25 PM
ImageData is there to be able to manipulate the raw pixels (the data attribute is a PixelArray object, with the setter and getter).

When you add the DXCanvas ability to a desktopX object, it completly takes over the painting for that object, so you don't need to set any object property to alter the object appearance - you simply make calls to the api. Does that answer your question?

And thanks for the kind words!
Reply #5 Tuesday, August 26, 2008 12:41 PM
Yep, answers my question...But generates a new one. I assume that loadImage(path)will allow you to load an external image into the context (replacing DX's object.picture = "c:\myimage.png"), then the drawing functions would draw 'on' the loaded image?
Reply #6 Tuesday, August 26, 2008 1:06 PM
Not exactly. loadImage will load an image and encapsulate it into a CanvasImage object. You will then be able to pass that object to createPattern and drawImage.

From script, you will simply do something like this:

var ctx = canvas.getContext("2d");

// load an image
var img = ctx.loadImage(Object.Directory + "image.png");

// Draw the image at context coordinates (5,5)
ctx.drawImage(img, 5, 5);

See this tutorial on using images.

Of course, you can also create a pattern using this image and then draw a rectangle with this pattern as a fill or stroke style.

When you create a pattern (be it gradient, radial or image/canvas-based) or load an image, you get an object back that can be used in latter drawing steps. But it doesn't draw anything on the context at that time.
Reply #7 Tuesday, August 26, 2008 9:47 PM
Nice work. Did a couple of calendar scripts which clarified the need for text like you said. Guess it depends on how interactive you want things.

Not sure if I borked something or it's how your plugin works but, scripting the cursor seems to be inconsistent. A good example is the navigation script within these three scripts I uploaded Click me, you know you want to!
Reply #8 Tuesday, August 26, 2008 11:27 PM
Not sure if I borked something or it's how your plugin works but, scripting the cursor seems to be inconsistent.

I'm not intercepting any window messages yet (although I will probably add a configuration option in the future, so you can drag things around the canvas), so there should not be any impact on how DesktopX functions work.

I've been working on image and patterns today, but text is coming after that.

It's getting a bit late here (plus I worked through last night) and I'm not seeing any obvious problem with the script. Can you point to me what exactly is not working correctly (and how it should behave)?

Thanks for the feedback (and for writing a few scripts to add to my test collection )!
Reply #9 Wednesday, August 27, 2008 12:20 PM
Did I mention that this Rocks!
Reply #10 Wednesday, August 27, 2008 2:21 PM
You're welcome. The scripts are the least I can to do help out.

Can you point to me what exactly is not working correctly (and how it should behave)?

When you load the Navigation script, the cursor should change to hand when you mouse over the left/right edges (10px) then when you leave those regions the cursor should return to default.

I even tried scripting x,y with the mouse enter/leave functions with the same effects. Actually, I think this may be a DX issue because essentially what I'm trying to do is script regions. A timer should resolve this.
Reply #11 Wednesday, August 27, 2008 3:10 PM
Ah, but OnMouseEnter and OnMouseLeave are only called when the mouse enters or leaves the object, so you won't be able to update the cursor per region. Does this work with a standard object?

Using OnMouseMove(x, y) should work.

'Called on mouse over object
Function Object_OnMouseMove(x, y)
If x < 10 Then
Object.Cursor = 14
ElseIf x > Object.Width-10 Then
Object.Cursor = 14
ElseIf x > 10 And x < Object.Width-10 Then
Object.Cursor = 0
End If
End Function
Reply #12 Wednesday, August 27, 2008 6:20 PM
I don't think it does as this is why I use object overlay's. A refresh object command would be a nice option but, using a timer triggered on focus might do it.
Reply #13 Thursday, August 28, 2008 1:23 AM
New build with image support!

Also updated the canvas controller to support subfolders.

And drawing is still flickering like mad...

Reply #14 Thursday, August 28, 2008 12:54 PM
Text! That is if boxes qualify as text

The cairo text api (aptly named the toy text api) wasn't going to cut it, so I switched to pango, which has a cairo path for drawing text. And oy, what a mess. There are so many dependencies, it's not even funny. Of course, the win32 makefiles are badly broken, and forget about getting a static lib to build without diving into the code.

After a lot of fumbling with all those libs, I finally got it to build with all its dependencies. It's know processing the text using the fill or stroke style, but it fails to get any font, so it draws boxes instead...

Turns out I might be missing some engine module (dynamically loaded, but you can link them statically apparently). Gah, yet more work!
Reply #15 Thursday, August 28, 2008 1:28 PM

I can't get the latest build to draw. The Canvas Controller loads, populates the listbox with scripts, but nothing happens when you select a script and the 'ok'
Reply #16 Thursday, August 28, 2008 1:41 PM
Try re-setting the script directory to the new location. I changed the application folder name and it might still point to the old location. Sorry about those changes! For the next version, you should be able to overwrite the previous one and it will continue to work.
Reply #17 Thursday, August 28, 2008 1:49 PM
I noticed that cairo.dll is missing from the new version zip. I copied it over from the original version but still nothing, even after resetting the script directory. The original Canvas Controller still works.
Reply #18 Thursday, August 28, 2008 2:27 PM
cairo.dll is not needed anymore. The plugin is self-contained now.

Hum, maybe I know what's happening: I might have exported it from my second monitor and it's showing off screen. I'll add a reset icon in the next build

You can redownloaded the package, it should be working now.

Reply #19 Thursday, August 28, 2008 2:43 PM
Thanks! It works now.
Reply #20 Friday, August 29, 2008 9:53 PM

It still needs some tweaking but it's better than squares

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